《SpringBoot》自动装配原理（简单易懂）-编程知识

《SpringBoot》自动装配原理（简单易懂）

news/2025/1/20 8:19:25/文章来源:https://www.cnblogs.com/seven97-top/p/18679465

引入

先看SpringBoot的主配置类

@SpringBootApplication
public class DemoApplication{public static void main(String[] args){SpringApplication.run(StartEurekaApplication.class, args);}
}

@SpringBootApplication

点进@SpringBootApplication来看，发现@SpringBootApplication是一个组合注解。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {}

@SpringBootApplication 由 @Configuration、@EnableAutoConfiguration、@ComponentScan 注解的集合组成：

@Configuration：允许注册额外的 bean 或导入其他配置类
@EnableAutoConfiguration：启用 SpringBoot 的自动配置机制
@ComponentScan：扫描被@Component (@Repository,@Service,@Controller)注解的 bean，注解默认会扫描该类所在的包下所有的类。

@SpringBootConfiguration

@SpringBootConfiguration 注解源码如下：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

可以看到这个注解除了元注解以外，就只有一个@Configuration，那也就是说这个注解相当于@Configuration，所以这两个注解作用是一样的，也就是能够去注册一些额外的Bean，并且导入一些额外的配置。

@Configuration还有一个作用就是把该类变成一个配置类，不需要额外的XML进行配置。所以@SpringBootConfiguration就相当于@Configuration。

进入@Configuration，发现@Configuration核心是@Component，说明Spring的配置类也是Spring的一个组件。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {@AliasFor(annotation = Component.class)String value() default "";
}

@EnableAutoConfiguration

继续看@EnableAutoConfiguration，这个注解是开启自动配置的功能，源码如下：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";Class<?>[] exclude() default {};String[] excludeName() default {};
}

可以看到它是由 @AutoConfigurationPackage，@Import(EnableAutoConfigurationImportSelector.class)这两个而组成的，

@AutoConfigurationPackage

先看@AutoConfigurationPackage，这是为了让包中的类以及子包中的类能够被自动扫描到spring容器中。

源码如下：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import({Registrar.class})
public @interface AutoConfigurationPackage {
}

可以看到，这里使用@Import 来给Spring容器中导入一个组件，这里导入的是Registrar.class。来看下这个Registrar：

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {Registrar() {}public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName());}public Set<Object> determineImports(AnnotationMetadata metadata) {return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata));}}

就是通过以上这个方法获取扫描的包路径，可以debug查看具体的值：

那metadata是什么呢，可以看到是标注在@SpringBootApplication注解上的DemoApplication，也就是主配置类Application：

其实就是将主配置类（即@SpringBootApplication标注的类）的所在包及子包里面所有组件扫描加载到Spring容器。因此要把DemoApplication放在项目的最高级中（最外层目录）。

@Import(AutoConfigurationImportSelector.class)

看看注解@Import(AutoConfigurationImportSelector.class)，@Import注解就是给Spring容器中导入一些组件，这里传入了一个组件的选择器:AutoConfigurationImportSelector。

可以从图中看出AutoConfigurationImportSelector 继承了 DeferredImportSelector 继承了 ImportSelector，ImportSelector有一个方法为：selectImports。将所有需要导入的组件以全类名的方式返回，这些组件就会被添加到容器中。

public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return NO_IMPORTS;} else {AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}
}

这里会给容器中导入自动配置类（xxxAutoConfiguration），也就是给容器中导入这个场景需要的所有组件，并配置好这些组件。

有了自动配置类，就免去了手动编写配置注入功能组件等的工作。

那是如何获取到这些配置类的呢，看看下面这个方法：

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return EMPTY_ENTRY;} else {AnnotationAttributes attributes = this.getAttributes(annotationMetadata);List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);configurations = this.removeDuplicates(configurations);Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);this.checkExcludedClasses(configurations, exclusions);configurations.removeAll(exclusions);configurations = this.filter(configurations, autoConfigurationMetadata);this.fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);}
}

可以看到getCandidateConfigurations()这个方法，他的作用就是引入系统已经加载好的一些类，那么到底是那些类呢：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");return configurations;
}
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {String factoryClassName = factoryClass.getName();return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}

会从META-INF/spring.factories中获取资源，然后通过Properties加载资源：

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);if (result != null) {return result;} else {try {Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");LinkedMultiValueMap result = new LinkedMultiValueMap();while(urls.hasMoreElements()) {URL url = (URL)urls.nextElement();UrlResource resource = new UrlResource(url);Properties properties = PropertiesLoaderUtils.loadProperties(resource);Iterator var6 = properties.entrySet().iterator();while(var6.hasNext()) {Map.Entry<?, ?> entry = (Map.Entry)var6.next();String factoryClassName = ((String)entry.getKey()).trim();String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());int var10 = var9.length;for(int var11 = 0; var11 < var10; ++var11) {String factoryName = var9[var11];result.add(factoryClassName, factoryName.trim());}}}cache.put(classLoader, result);return result;} catch (IOException var13) {throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);}}
}

可以知道SpringBoot在启动的时候从类路径下的META-INF/spring.factories中获取EnableAutoConfiguration指定的值，将这些值作为自动配置类导入到容器中，自动配置类就生效，帮我们进行自动配置工作。以前需要自己配置的东西，自动配置类都帮我们完成了。

如下图可以发现Spring常见的一些类已经自动导入。

@ComponentScan

接下来看@ComponentScan注解，@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })，这个注解就是扫描包，然后放入spring容器。

@ComponentScan(excludeFilters = {@Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}), @Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class})})
public @interface SpringBootApplication {}

总结下@SpringbootApplication：就是说，他已经把很多东西准备好，具体是否使用取决于我们的程序或者说配置。

小结

总的来说，SpringBoot的自动装配原理就是通过@EnableAutoConfiguration注解在类路径的META-INF/spring.factories文件中找到所有的对应配置类，然后将这些自动配置类加载到spring容器中

run方法

public static void main(String[] args) {SpringApplication.run(Application.class, args);}

来看下在执行run方法到底有没有用到哪些自动配置的东西，点进run：

public ConfigurableApplicationContext run(String... args) {//计时器StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();this.configureHeadlessProperty();//监听器SpringApplicationRunListeners listeners = this.getRunListeners(args);listeners.starting();Collection exceptionReporters;try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);this.configureIgnoreBeanInfo(environment);Banner printedBanner = this.printBanner(environment);//准备上下文context = this.createApplicationContext();exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class,                       new Class[]{ConfigurableApplicationContext.class}, context);//预刷新contextthis.prepareContext(context, environment, listeners, applicationArguments, printedBanner);//刷新contextthis.refreshContext(context);//刷新之后的contextthis.afterRefresh(context, applicationArguments);stopWatch.stop();if (this.logStartupInfo) {(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);}listeners.started(context);this.callRunners(context, applicationArguments);} catch (Throwable var10) {this.handleRunFailure(context, var10, exceptionReporters, listeners);throw new IllegalStateException(var10);}try {listeners.running(context);return context;} catch (Throwable var9) {this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);throw new IllegalStateException(var9);}
}

那我们关注的就是 refreshContext(context); 刷新context，我们点进来看。

private void refreshContext(ConfigurableApplicationContext context) {refresh(context);if (this.registerShutdownHook) {try {context.registerShutdownHook();}catch (AccessControlException ex) {// Not allowed in some environments.}}
}

继续点进refresh(context);

protected void refresh(ApplicationContext applicationContext) {Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);((AbstractApplicationContext) applicationContext).refresh();
}

会调用 ((AbstractApplicationContext) applicationContext).refresh();方法，点进来看：

public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.prepareRefresh();// Tell the subclass to refresh the internal bean factory.ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.registerBeanPostProcessors(beanFactory);// Initialize message source for this context.initMessageSource();// Initialize event multicaster for this context.initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.onRefresh();// Check for listener beans and register them.registerListeners();// Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.finishRefresh();}catch (BeansException ex) {if (logger.isWarnEnabled()) {logger.warn("Exception encountered during context initialization - " +"cancelling refresh attempt: " + ex);}// Destroy already created singletons to avoid dangling resources.destroyBeans();// Reset 'active' flag.cancelRefresh(ex);// Propagate exception to caller.throw ex;}finally {// Reset common introspection caches in Spring's core, since we// might not ever need metadata for singleton beans anymore...resetCommonCaches();}}
}

由此可知，就是一个spring的bean的加载过程。继续来看一个方法叫做 onRefresh()：

protected void onRefresh() throws BeansException {// For subclasses: do nothing by default.
}

在这里并没有直接实现，找他的具体实现：

比如Tomcat跟web有关，可以看到有个ServletWebServerApplicationContext：

@Override
protected void onRefresh() {super.onRefresh();try {createWebServer();}catch (Throwable ex) {throw new ApplicationContextException("Unable to start web server", ex);}
}

可以看到有一个createWebServer()方法，用于创建web容器，而Tomcat不就是web容器。

那是如何创建的呢：

private void createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = getServletContext();if (webServer == null && servletContext == null) {ServletWebServerFactory factory = getWebServerFactory();this.webServer = factory.getWebServer(getSelfInitializer());}else if (servletContext != null) {try {getSelfInitializer().onStartup(servletContext);}catch (ServletException ex) {throw new ApplicationContextException("Cannot initialize servlet context",ex);}}initPropertySources();
}

factory.getWebServer(getSelfInitializer())，显然是通过工厂的方式创建的。

public interface ServletWebServerFactory {WebServer getWebServer(ServletContextInitializer... initializers);
}

可以看到它是一个接口，为什么会是接口。因为不止是Tomcat一种web容器，可以看到还有Jetty

接下来看TomcatServletWebServerFactory：

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {Tomcat tomcat = new Tomcat();File baseDir = (this.baseDirectory != null) ? this.baseDirectory: createTempDir("tomcat");tomcat.setBaseDir(baseDir.getAbsolutePath());Connector connector = new Connector(this.protocol);tomcat.getService().addConnector(connector);customizeConnector(connector);tomcat.setConnector(connector);tomcat.getHost().setAutoDeploy(false);configureEngine(tomcat.getEngine());for (Connector additionalConnector : this.additionalTomcatConnectors) {tomcat.getService().addConnector(additionalConnector);}prepareContext(tomcat.getHost(), initializers);return getTomcatWebServer(tomcat);
}

这块代码，就是要寻找的内置Tomcat，在这个过程当中，可以看到创建Tomcat的一个流程。

也就是：